Acoustic hydrophones are commonly employed in ASW and seismic applications to convert acoustic waves in a fluid medium such as sea water, to signals representative of sound waves in the fluid. A hydrophone typically includes a wet-end sensor which is disposed in the fluid, a dry-end signal processor, and a cable for connection of the sensor to the processor. The signal processor is usually located aboard a ship or a submarine and the wet-end sensor is towed behind the vessel.
One type of acoustic hydrophone utilizes an electromechanical sensor such as a magnetostrictive or piezoelectric transducer as the wet-end sensor. Such sensors are connected to a remote signal processor via an electrical cable. The use of an electrical cable for connection of the sensor to the processor is disfavored due to problems such as cable corrosion, cable breakage, electrical cross-talk, cable bulk and expense.
Optoacoustic hydrophones which employ fiber optic leads and a fiber optic sensor have become increasingly common in the last few years contemporaneous with technological advances in fiber optic technology. The use of fiber optic leads for connection of the wet-end sensor to the dry-end processor avoids the problems associated with the use of electrical cables.
Typical optoacoustic hydrophones employ a fiber optic sensor as the wet-end sensor and utilize fiber optic leads for connection of the sensor to the remote processor. Coherent light at a first frequency is directed through the leads and the sensor. Acoustic waves impinge on the fiber optic coil and modulate the coil optical path lengths thereby modulating the transmitted light beam. The modulated light beam is combined with coherent light of a slightly different frequency to produce a modulated interference pattern which is converted to an electrical signal representative of the impinging acoustic waves.
It has been observed that towage of the fiber optic leads through the fluid produces undesirable modulation of the transmitted light beam resulting from fluid acoustic noise interactions with the fiber optic leads. The lead related modulation results in distortion of the detected signal which is due partially to flow noise.
Monsay and Gilbert in a paper entitled "Predicted Performance of a Heterodyne Detector with a Fiber Optic Coil Hydrophone" describe a fiber optic hydrophone which minimizes distortion of acoustic signals resulting from modulation of the lead fibers due to stray acoustic signals and flow noise. The Monsay hydrophone employs a heterodyne detection system in which two coherent light beams at slightly different frequencies are focussed into two optical fibers which are optically coupled to the wet-end fiber optic sensor. At the sensor, each lead fiber end is coated with a dielectric mirror coating to partially reflect light back down respective leads. Acoustic waves in the fluid modulate the index of refraction of the coil. The transmitted portions of the two coherent light beams are modulated at the sensor. The light beams exit the respective lead fibers and each beam is combined with another light beam of slightly different frequency to produce two interference patterns modulated in accordance with the acoustic wave impinging the sensor. The phase difference between the two interference signals is measured to produce a signal representative of the acoustic wave alone.
Bucaro in U.S. Pat. No. 4,162,397 discloses another fiber optic acoustic sensor for detection of sound waves in a fluid medium. Bucaro shows a fiber optic coil sensor disposed in one path of a double path interferometer, the second path being isolated from the acoustic field. Acoustic waves impinging on the fiber optic coil modulate the coil's optical path length and modulate a first light beam transmitted through the coil. A second beam of light is directed along an isolated and unmodulated path. The two beams are combined to produce an interference pattern which is directed into a photocell. The photocell produces a signal which is filtered to provide an output signal representative of the acoustic wave.
In prior fiber optic hydrophones, trade offs were made between hydrophone linearity and the magnitude of the signal to noise ratio. It has been necessary to employ lead fibers having a low transmission efficiency to avoid multiple reflections of light at the sensor boundaries with consequent distortion of the detected signal. Additionally, it has been necessary to maintain a low light beam intensity to avoid multiple beam reflections and accompanying signal distortion.